The printing system to which this invention is directed and current control systems for the printhead are disclosed in U.S. Pat. No. 4,350,449 to Countryman et al and U.S. Pat. No. 4,345,845 to A. E. Bohnhoff et al, which are herein incorporated by reference. U.S. Pat. No. 4,350,449 teaches constant-current driver circuits driving each of the electrodes. The system disclosed drives each electrode from a fixed potential. Where it is desirable to miniaturize the circuit by building it primarily on a substrate (chip), dissipation of power delivered by the fixed potential is a factor because it tends to require off-chip elements. This patent also discloses that the voltage level at the area of printing shifts for each different number of electrodes driven, a factor potentially increasing heat production which the invention of this application neutralizes.
U.S. Pat. No. 4,345,845 teaches a monitoring contact spaced from the printhead a distance in a direction opposite from the grounding contact. The signal from that monitoring contact is compared with the reference signal and all of the driving currents are created in single circuit based on that comparison. The patent thus teaches one solution to the problem of varying electrical characteristics at the ribbon during ordinary operation.
Another teaching in which separate driver circuits are connected to each electrode is found in IBM Technical Disclosure Bulletin article entitled "Constant Current (Current Source) Resistive Ribbon Print Head Array Drive Scheme" by G. P. Countryman and R. G. Findlay, Vol. 22, No. 2, July 1979, at pp. 790-791. This article shows fixed-drive potential, constant current circuit arrangements closely similar to those of the foregoing U.S. Pat. No. 4,350,449.
A number of prior art teachings might be cited showing printheads driven with systems which are regulated to adjust to printing-related factors such as temperature at the point of printing, time delays between closely spaced printing, and other such factors. This invention is concerned with the variations in voltage level at the contact of the printhead to a resistive ribbon, and no prior art teaching or the like other than the foregoing Bohnhoff patent is known to directly monitor and react to changes in that voltage level. As does the Bohnhoff patent, this invention obtains a single signal which is employed to adjust the input current to all of the driven electrodes. This single signal, however, in distinction to that in Bohnhoff, is obtained directly at the electrodes. That single signal is used to control the operating level of a plurality of constant-current drivers, one for each electrode.